Acceptor and Donor Dopants in Potassium Sodium Niobate Based Ceramics

Hussain, Fayaz; Khesro, Amir; Lu, Zhilun; Alotaibi, Nouf; Mohamad, Ahmad Azmin; Wang, Ge; Wang, Dawei; Zhou, Di

doi:10.3389/fmats.2020.00160

ORIGINAL RESEARCH article

Front. Mater., 08 July 2020
Sec. Ceramics and Glass
Volume 7 - 2020 | https://doi.org/10.3389/fmats.2020.00160

Acceptor and Donor Dopants in Potassium Sodium Niobate Based Ceramics

Fayaz Hussain^1,2*

Amir Khesro³

Zhilun Lu^2,4

Nouf Alotaibi⁵

Ahmad Azmin Mohamad⁶

Ge Wang²

Dawei Wang²

Di Zhou^7*

¹Department of Materials Engineering, NED University of Engineering & Technology, Karachi, Pakistan
²Department of Material Science and Engineering, University of Sheffield, Sheffield, United Kingdom
³Department of Physics, Abdul Wali Khan University, Mardan, Pakistan
⁴The Henry Royce Institute, Sheffield, United Kingdom
⁵Chemistry Department, King Saud University, Riyadh, Saudi Arabia
⁶School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia
⁷Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, China

B-site doping in potassium sodium niobate (KNN) with Mn²⁺ ( $M n_{Nb}^{″'}$ ) and Ti⁴⁺ ( $T i_{Nb}^{'}$ ) dopants were soluble but prevented KNN from achieving a high relative density, while Sn⁴⁺ ( $S n_{Nb}^{'}$ ) was not soluble in the structure as evidenced by second phase peaks in X-ray diffraction (XRD) traces. However, SnO₂ was an effective sintering aid in KNN-50/50. A-site doping with Sr²⁺ ( $S r_{(Na, K)}^{\cdot}$ ) up to 1 mol% initially improved dielectric properties but higher sintering temperatures were required for compositions with >1 mol% Sr. Samples with 5% and 7% of Sr-doping completely shifted the transition of T_O–T to below RT and broadened the T_C peaks as the relaxor. All Ti-doped and Sr-doped compositions showed an increase in conductivity, manifested as high values of dielectric loss (tanδ). More than 1% of acceptor and donor dopants showed the ionic-type conduction mechanism, while 1% displayed the electronic mechanism as attributed from the strongly frequency-dependent tanδ. In conclusion, these samples have the potential to open up new applications in the field of electroceramics.

Introduction

Different properties and effects of dopants on potassium sodium niobate (KNN)-based ceramics have been discussed in many publications (Lin et al., 2007; Lee et al., 2008; Wang et al., 2008; Liu et al., 2009, 2012; Tan et al., 2012; Zhao et al., 2013; Bafandeh et al., 2014; Wu J. et al., 2014; Zheng et al., 2015; Yang et al., 2016; Hussain et al., 2019, 2020). Some groups have discussed potassium oxide and sodium oxide both separately and with doping elements, in terms of electrical properties for KNN-based ceramics (Jaffe et al., 1971; Fluckiger and Arend, 1978; Kodaira et al., 1982; Jenko et al., 2005; Lee et al., 2008; Wang et al., 2008; Zhang et al., 2013; Zhao et al., 2013; Zheng et al., 2015). Fewer studies have been done on acceptor (Akça and Yılmaz, 2015; Rafiq et al., 2015; Chen et al., 2016) and donor (Wu S. et al., 2014; Hreščak et al., 2017) dopants in KNN to understand its electrical properties. Donor and acceptor dopants play a vital role in the investigation into the semiconducting properties and defect chemistry of the well-established systems of Barium Titanate (BT) and Lead Zirconate Titanate (PZT; Masó et al., 2006; Da-Wei et al., 2009; Cao et al., 2010; Erdem et al., 2010; Wang D. W. et al., 2011; Wang et al., 2012, 2014; Wang D. et al., 2013; Ma et al., 2012; Zhu et al., 2012; Ali et al., 2013; Freeman et al., 2013; Li et al., 2013, 2015; Lou et al., 2018). Hreščak et al. (2017) reported Sr²⁺ as a donor dopant in KNN that improved its crystal structure, grain size, and phase composition. Rafiq et al. (2015) reported that the single acceptor-dopant of Mn²⁺ in KNN decreased the leakage current; most authors, however, have emphasized the importance of co-doping (Guo et al., 2004, 2005; Hollenstein et al., 2005; Hollenstein et al., 2007; Jiang et al., 2007; Lopez-Juarez et al., 2011; Rubio-Marcos et al., 2011; Skidmore and Milne, 2011; Wang H. Q. et al., 2011; Wang K. et al., 2013; Wang et al., 2017; Zhang et al., 2011; Li et al., 2012; Du et al., 2012a, b; Gio and Phong, 2015; Liu et al., 2016, 2017) in achieving optimized functional properties. Chen et al. (2016) prepared different formulations with trace amounts of acceptor-dopants into (K₀.₅Na₀.₅ Nb₀.₉₉₄A₀.₀₀₆)O_3–δ, where A(Ga³⁺, Ge⁴⁺, Mn²⁺, Zn²⁺, Cu²⁺, and Ni²⁺) metallic-ions were incorporated; this consequently improved the high mechanical-quality-factor (Q_m) values in divalent-doped compositions.

The aim of this research was to introduce acceptor-dopants (Mn²⁺, Ti⁴⁺, and Sn⁴⁺) and donor-dopants (Sr²⁺) into KNN to see the individual effect of their solubility and structural changes on the electrical properties of KNN-50/50-based ceramics.

Experimental Procedure

Nb₂O₅ was obtained from Stanford Materials Corporation with 99.999% purity, whereas K₂CO₃ and Na₂CO₃ were obtained from Fisher Scientific with 99.9% anhydrous used. In addition to this, TiO₂ was obtained from Aldrich Chemistry with 99.99%, and MnO₂, SnO₂, and SrCO₃ were obtained from Aldrich with 99.9% purity each. Carbonates were dried at 300°C, whereas all other oxides were heated at 900°C for 24 h. Batches of 50 g of doped-KNN formulations (K₀.₅Na₀.₅Ti_xNb_1–xO_3–x/2 where 0.0 ≤ x ≤ 0.05; K₀.₅Na₀.₅Mn_xNb_1–xO_3–x/2 where 0.0 ≤ x ≤ 0.01; K₀.₅Na₀.₅Sn_xNb_1–xO_3–x/2 where 0.0 ≤ x ≤ 0.03; and (K₀.₅Na₀.₅)_1–xSr_xNbO₃ where 0.0 ≤ x ≤ 0.07) were prepared from dried raw powders in a hot condition (∼200°C) to avoid the non-stoichiometric conditions caused by moisture, especially in carbonates. All compositions were attrition milled for 1 h in a 500 ml jar at 300 rpm in isopropanol using 3 mm Dia. Y₂O₃ stabilized zirconia milling media prior to calcination. After milling, the slurry was washed with further isopropanol, separated from milling media through a sieve, and volatiles were removed at 80°C for 24 h in a drying oven. The dried material was sieved through a 150-micron mesh and calcined for 6 h at 850°C at 3°C/min and 5°C/min, heating and cooling rates, respectively. Reacted powders were re-milled before pressing a pellet. 10 mm diameter disks of all compounds were pressed uniaxially with 2-ton force and fired at in a temperature range of 1120–1165°C for 2–8 h. Moreover, the details of sintering temperatures of all formulations are described in Supplementary Figures S1, S2.

Densities of pellets were measured as per the Archimedes method, which were around 90%-bulk in average. XRD traces of sintered pellets were obtained using a Siemens D500 diffractometer at the 2θ range of 10^o–80^o, using CuKα radiation. The dielectric properties were characterised using an LCR meter (Model 4284A, Hewlett Packard).

Results and Discussion

XRD

The XRD traces from KNN-50/50 doped on the B-site with Mn²⁺, Ti⁴⁺, and Sn⁴⁺ are shown in Figures 1, 2. The XRD traces from KNN-50/50 doped with 1% Mn and 1% Ti both appear to be similar, but the shape of 2θ ∼ 45° {220}, and {002} peaks shows a slight difference (Figure 1). In both cases, the intensity of the 2θ = 32° {111} is lower than that of the undoped KNN (Lin et al., 2010). Increasing the concentration of Ti to 5% Ti emphasized the structural change observed at 1% Ti. There are a few studies on single Mn acceptor dopant in KNN. Peaks shifting toward higher 2θ is evidence of shrinkage in the lattice volume, which took place for Mn²⁺ (Rafiq et al., 2015) and Ti⁴⁺ acceptor dopants as compared to the undoped KNN peak (shown in Figure 4; 2θ = 45–46°). Conversely, Lin et al. (2010) reported that XRD peaks shifted toward lower 2θ (i.e., expansion in the lattice volume) in the case of Mn⁴⁺ doped KNN.

FIGURE 1

Figure 1. (A) Single phase XRD patterns of Mn- and Ti-doped KNN-50/50, (B) zoomed peaks at 2θ ∼ 45–46.

FIGURE 2

Figure 2. (A) XRD patterns of Sn-doped KNN-50/50, (B) zoomed peak at 2θ ∼ 45–46.

For KNN-50/50 doped with Sn⁴⁺, secondary peaks of SnO₂ are visible in Figure 2. Moreover, within the resolution limits of in-house XRD, there was no discernible change in the trace of the major KNN peaks, confirming that Sn does not enter into solid solution with KNN. Su et al. (2010) investigated SnO₂ and CuO co-doping in KNN and concluded that Sn⁴⁺ was not soluble after 1 mol%. Their XRD traces also depicted secondary peak positions similar to those in this study. Akça and Yılmaz (2015) also reported insolubility issues for Sn⁴⁺ in KNN with secondary peaks visible in their XRD data. The reasons behind the insolubility of Sn⁴⁺ issues in KNN is unclear, since Sn⁴⁺ (0.69 Å) has a similar ionic radius to Nb⁵⁺ (0.68 Å; Shannon, 1976). However, Sn⁴⁺ is more covalently bonded to O than Nb⁵⁺, which may influence its solubility (Barret, 1962; IBchem, 2016).

For KNN-50/50 doped with Sr²⁺ on the A-site, the XRD traces revealed a broadening of the {111} peaks as a function of x with respect to undoped KNN-50/50 when fired in both air and N₂. Figures 3, 4 should be evaluated together, indicating that Sr²⁺ was incorporated within the KNN lattice.

FIGURE 3

Figure 3. (A) X-ray diffraction data of Sr doped KNN-50/50, (B) zoomed peak at 2θ ∼ 45–46.

FIGURE 4

Figure 4. Zoomed peak at 2θ ∼ 45–46, in comparision with the peak shape of 1% Sr_air/N₂ with KNN_50/50_air.

Dielectric Properties

Acceptor Dopants: Mn²⁺, Ti⁴⁺, and Sn⁴⁺

The temperature dependence of relative permittivity (ε_r) and loss (tanδ) of 1 mol% Mn doped KNN-50/50 are shown in Figures 5A,B as a function of temperature and frequency. Two phase transition temperatures, i.e., orthorhombic-tetragonal (T_O–T), and tetragonal-cubic (T_T–C) are clearly presented, though both transitions are shifted toward lower temperatures compared to undoped KNN-50/50 sintered in air. The ε_r and tanδ at room temperature of 1% Mn doped KNN-50/50 was 380 and 0.05 at 100 kHz, respectively. However, at lower frequencies, the tanδ increased dramatically when increasing the temperature. The most likely cause of tanδ relates to oxygen vacancy (V $_{O}^{\cdot \cdot}$ ) formed according to the defect equation:

FIGURE 5

Figure 5. (A) ε_r and (B) tanδ versus temperature of Mn-doped KNN; (C) ε_r and (D) tanδ versus temperature of Ti-doped KNN compositions.

{Mn}_{Nb}^{″'} \equiv 3 / 2 V_{O}^{∙ ∙}

At high frequencies (1 MHz), tanδ was suppressed with acceptor Mn²⁺ dopant. At these frequencies, the loss mechanism relating to V $_{O}^{\cdot \cdot}$ may clamp out, resulting in a decrease in the overall tanδ. There are a number of potential loss mechanisms relating to V $_{O}^{\cdot \cdot}$ , such as rotation of defect dipoles and movement of space charge. It is not known which mechanism dominates in this study.

The dielectric properties of Ti-doped KNN-50/50 are shown in Figures 5C,D. For low concentrations, (1%) the T_T–C temperature decreased with little change in T_O–T. However, for high concentrations, T_T–C phase transition temperatures increased, and T_O–T decreased compared to the undoped and 1% Ti doped compositions and the phase transitions become broader. The peak in permittivity at around room temperature in 3 and 5% Ti-doped KNN may relate to the observation of minor changes to the shape of some XRD peaks (at {220} and {002} planes; Figure 1). Dielectric loss in Ti-doped suppressed from x = 0.01 to x = 0.05 with respect to the temperature and frequency, which suggested that the conduction mechanism shifted from electronic to ionic as the Ti dopant increased. Nevertheless, electronic species are lighter than ionic ones; that’s why ionic species need more activation energy and consequently higher conductions at higher temperatures (see Supplementary Data).

The dielectric properties of Sn⁴⁺ doped KNN are shown in Figures 6A,B. Unsurprisingly, there was no effect on the dielectric properties with respect to undoped compositions since there was no evidence that Sn⁴⁺ enters the KNN lattice.

FIGURE 6

Figure 6. (A) ε_r and (B) tanδ verses temperature of Sn-doped KNN; (C) ε_r and (D) tanδ verses temperature of Sr-doped KNN compositions.

Donor Dopant: Sr²⁺

Dielectric properties of Sr donor dopant (A-site) in KNN-50/50 are presented in Figures 6C,D. 1% Sr-doped KNN-50/50 compositions were sintered in both air and N₂ to compare the behavior with undoped in a previous study (Hussain et al., 2018) and acceptor doped compositions in the previous sections. However, for undoped KNN sintered in N₂, the transition temperature decreased in agreement with 1% Sr-doped KNN (N₂). N₂ sintered Sr²⁺ KNN becomes more conductive as compared to undoped KNN, presumably because Sr²⁺ donates extra electrons (h^⋅) and low pO₂ creates $V_{O}^{\cdot \cdot}$ . It has been proposed that the increase in conductivity contributes to the larger permittivity for N₂ sintered KNN-1 Sr. T_T–C with acceptor (Mn²⁺) and donor (Sr²⁺) in air are 390°C and 384°C, respectively, which suggests that disruption of the ferroelectric order to reduce T_T–C is greater for A-site Sr²⁺ doping in comparison to B-site Mn²⁺. At higher Sr concentrations, x = 0.05 and 0.07, the phase transitions broaden in temperature (inset of Figure 6C), consistent with the broadening of peaks in the XRD traces (Figure 3). In addition, the dielectric loss increased when samples became more conductive at higher than 1% Sr (see Supplementary Data). Nonetheless, this broadening and higher dielectric loss phenomenon with donor doping looks similar to the results reported in the case of W⁶⁺ B-site donor doping in KNN, investigated by Wu S. et al. (2014).

Conclusion

Acceptor dopants such as Mn²⁺ ( $M n_{Nb}^{″'}$ ) and Ti⁴⁺ ( $T i_{Nb}^{'}$ ) at the B-site of KNN were incorporated to modify properties, but they both inhibited densification of KNN. Nonetheless, both acceptor species were soluble in the lattice, as revealed by XRD. Dielectric losses increased dramatically at lower frequencies with increasing temperatures by using acceptors but were moderately lower at higher frequencies. A further B-site dopant (Sn⁴⁺) was also attempted but was insoluble, as evidenced by the appearance of secondary phase peaks in XRD data at low concentrations. Nevertheless, Sn⁴⁺ was an effective sintering aid in KNN-50/50 and improved its relative density. Ceramic density improved with 1 mol% Sr²⁺ but at a higher sintering temperature. Sr²⁺-doped formulations showed higher conductivity, which manifested itself in higher values of tanδ. T_C of KNN-1 Sr in N₂ decreased with respect to air-sintered samples.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

FH: Investigation, software, data analysis, original draft writing, and critical discussion of results, etc. AK: Formal discussion, review and editing, and language. ZL: Conceptualization, review, and editing. NA, GW, and DZ: Review, editing, and technical additions. AM: Review, editing, and technical discussion. DW: Review, editing, formal analysis, and critical discussion. All authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

FH acknowledges NED University of Engineering and Technology, Pakistan, for funding support. All authors acknowledge financial help from the Sustainability and Substitution of Functional Materials and Devices EPSRC grant (EP/L017563/1) through Prof. Ian M. Reaney. The data of this work is from Ph.D. thesis of FH (Corresponding Author; Hussain, 2016). ZL acknowledges the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P02470X/1, and EP/P025285/1.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2020.00160/full#supplementary-material

References

Akça, E., and Yılmaz, H. (2015). Sintering behavior and electrical properties of K4CuNb8O23 modified K0.5Na0.5NbO3 ceramics with SnO2, ZnO or Yb2O3 doping. Ceramics Int. 41, 3659–3667. doi: 10.1016/j.ceramint.2014.11.035

ORIGINAL RESEARCH article

Acceptor and Donor Dopants in Potassium Sodium Niobate Based Ceramics

Introduction

Experimental Procedure

Results and Discussion

XRD

Dielectric Properties

Acceptor Dopants: Mn2+, Ti4+, and Sn4+

Donor Dopant: Sr2+

Conclusion

Data Availability Statement

Author Contributions

Conflict of Interest

Funding

Supplementary Material

References

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